Update xunit.js

Changed testsuite Attribute "duration" to "time".
Jenkins does not parse the generated xunit Files otherways.

See also:
https://groups.google.com/d/msg/casperjs/n1tbi0K9Tnk/MA4SO_4SImUJ

The XSD Schema:
http://windyroad.com.au/dl/Open%20Source/JUnit.xsd

@@ -153,7 +153,7 @@ XUnitExporter.prototype.getXML = function getXML() {
         suiteNode.appendChild(warningNode);
         this._xml.appendChild(suiteNode);
     }.bind(this));
-    this._xml.setAttribute('duration', utils.ms2seconds(this.results.calculateDuration()));
+    this._xml.setAttribute('time', utils.ms2seconds(this.results.calculateDuration()));
     return this._xml;
 };